In raster electron microscopes and other particle-optical raster apparatus, the positioning of the object takes place relative to the particle-optical or electron-optical beam axis with the aid of the object table which is adjustable in three mutually perpendicular directions. A CCD camera having a camera illumination serves for viewing the object during the positioning. The camera illumination is often infrared. The optic of the CCD camera has a very high depth of focus. For this reason, it is hardly possible to position the object in the direction of the optical axis of the CCD camera with an accuracy of better than approximately 5 mm. With a subsequent switchover to electron irradiation or particle-optical irradiation and a recordation of an image of the object with the aid of the secondary particles, which are released by particle radiation, it can happen that even for an adjusted low magnification, the desired object detail is not seen in the image. This is so because the region, which is scanned by the particle beam, is smaller than the positioning accuracy in the image of the CCD camera and therefore the object region of interest still lies outside of the region scanned by the particle beam. The above-mentioned secondary particles are, for example, backscattered electrons or secondary electrons or light quanta which are released from the object itself or arise from the interaction of backscattered electrons or secondary electrons with gas molecules in the chamber.
Furthermore, the CCD camera image functions also for positioning the object in the direction of the particle beam axis, that is, for bringing the object plane into the plane defined by the work distance of the particle-optical objective. For an inexperienced user, this is likewise possible only with an accuracy of a few millimeters which is often not sufficient for a particle-optical image focused also only coarsely. For this reason, it is often necessary to seek the desired position on the object as well as to adjust the object to the correct work distance under particle-optical irradiation and the position sensitivity which occurs thereby.
In JP 63254649, it has already been suggested to provide, in addition, an optical microscope in a raster electron microscope and, with the laser beam thereof, to generate an ancillary signal for the generation of an autofocus signal for the electron optic.
Further relevant state of the art for the present invention includes: JP 2004-4319518, JP 10352776, JP 03141544, U.S. Pat. No. 6,714,289 and U.S. patent application publication US 2003/0006372.